Technological Field
The present invention relates to a developing device and an image forming apparatus.
Description of the Related Art
In a conventional image forming apparatus such as a copying machine, a printer, or the like, it is known that a trickle development method can be adopted. By the trickle development method, a new developer is supplied to a developing device that houses a two-component developer (hereinafter simply referred to as a “developer”) containing toner and a carrier (see JP 2012-173536 A, for example).
The trickle development method is designed for replenishing the inside of a developing device with a new developer, and discharging part of the developer housed in the developing device to the outside of the developing device, so that the number of degraded carriers in the developing device is reduced, and the amount and the charging ability of the carrier housed in the developing device are maintained. With an image forming apparatus using such a trickle development method, it is possible to achieve stable output image quality.
In a developing device adopting the trickle development method, however, the amount of the developer in the developing device greatly fluctuates depending on coverage, environmental temperature and humidity, the installation state of the developing device, and the like. Because of this, there is a problem that the toner concentration in the developer is not stabilized, and image defects such as fogging and image density fluctuations are caused. Further, in a developing device adopting the trickle development method, there is a problem that the necessary amount of the developer cannot be supplied to the developing roller, resulting in image unevenness. Particularly, in small-sized developing devices and developing devices to be driven at high speed in the field of production printing, it is difficult to stabilize the amount of the developer in a developing device, and the above problems are easily caused.
To counter the above problems, the technique disclosed in JP 2012-173536 A is designed to provide a carrier concentration detecting means in a developing device, predict the amount of the carrier to be injected into the developing device from the detected carrier concentration and the developer replenishment time, and control the rotation speed or the like of the stirring member in accordance with the predicted value. However, it is considered that the technique disclosed in JP 2012-173536 A does not provide a structure for maintaining the amount of the developer at a constant value in the developing device, and it is difficult to prevent the amount of the developer from fluctuating. Further, it is considered that a change in the rotation speed of the stirring member affects toner concentration control and, eventually, the quality of printed images, and therefore, such a change should not be frequently made.